Prostate cancer is prevalent in old age, with approximately one-half of all males over the age of 70 having been shown to develop prostatic cancer. This high incidence rate has led to the search for markers that may be useful in the detection of prostate malignancy. For example, serum acid phosphatase activity is elevated in patients having metastasized prostate carcinoma. Gutman et al., J. Clin. Invest. 17:473 (1938). Numerous studies of this enzyme and its relation to prostatic cancer have been made, but attempts to develop a specific test for prostatic acid phosphatase have met with only limited success. One problem is that there are difficulties associated with accounting for biological and immunological activity of unrelated phosphatases. Immunologic assays suffer additional limitations in terms of sensitivity.
Prostate specific antigen, or PSA, has been studied as another possible serum marker for prostatic cancer. The serum concentrations of PSA correlate with the clinical stage of prostatic cancer in untreated patients and are increased in 86% of patients with benign prostatic hyperplasia. PSA, also characterized independently as seminal plasma protein p30 (Sensabaugh and Blake, J. Urol. (UNITED STATES) December 1990, 144 (6) p1523-6) or as [.gamma.]-seminoprotein, (Hara and Kimura J. Lab. Clin. Med. (UNITED STATES) May 1989, 113 (5( p541-8), is a glycoprotein of 33-34 kDa. PSA is synthesized in, and secreted by, the epithelia cells of the prostate gland. It belongs to the group serine proteases, and the high molecular weight seminal vesicle protein of seminal coagulates is thought to be its normal biological substrate. Elucidation of the amino acid sequence of mature human PSA by sequencing of the protein and corresponding cDNA showed that PSA closely resembles many murine and human proteases of the kallikrein family.
Testing for PSA expression has become an important method in diagnosing and managing prostate cancer. The test is widely used, along with digital rectal examination and diagnostic ultrasonography, to permit an early diagnosis of prostate cancer in high-risk men. After radical prostatectomy, a fall in the serum PSA concentration to undetectable levels gives assurance of curative therapy, while measurable PSA concentrations which persist after surgery indicate the existence of extra-capsular disease and the need for additional therapy. Serial PSA monitoring of patients after curative therapy can identify patients who develop recurrent disease, in many cases long before clinical symptoms are evident.
Because of its potential as a diagnostic marker for human prostate cancer, it will be of interest to develop immunologic reagents capable of reacting specifically with PSA. In light of the structural relationship between PSA and other molecules, however, it appears that it will be necessary to generate antibodies against fragments of PSA, rather than against the entire molecule. Fragments can be generated by proteolytic cleavage but, because the sequence of PSA is known, it also is possible to chemically synthesize oligopeptides of interest.
Unfortunately, the ability of a given oligopeptide to elicit antibody responses that cross-react with the native molecule currently is unpredictable at best. One reason is that oligopeptides only have the ability to represent linear or "continuous" epitopes. "Discontinuous" epitopes are composed of sequences from throughout an antigen and rely on folding of the protein to bring the sequences into close proximity of one another. Clearly, oligopeptides are incapable of representing such epitopes. Moreover, even though continuous epitopes are structurally less complicated than discontinuous ones, there remains only a poor understanding of how the immune system recognizes and responds to these antigenic species.
The foregoing limitations are significant in that the clinical accuracy of a immunodiagnostic PSA assay is related to the specificity and sensitivity of the test procedure. Recently, it has been demonstrated that the existing monoclonal antibodies and polyclonal antisera used for current PSA assays cross-react with HgK-1. This raises serious questions regarding the accuracy of these tests and highlights the importance of developing more selective reagents for this kind of assay.
Unfortunately, the ability to predict which portions of a given antigen reflect epitopes presently is limited. On the other hand, random screening of oligopeptides to determine potential immunogens is a time-consuming and expensive endeavor that is impractical with even moderately-sized polypeptides. Thus, the identification of immunologic oligopeptides of PSA suitable for use in the development of PSA-specific antibodies is of paramount importance.